Oxygen transport membrane elements are used within reactors for gas separation and in the production of synthesis gas. They are manufactured from ceramic materials that conduct oxygen ions at elevated temperatures. On one surface of the membrane, known as the cathode side, oxygen in an oxygen containing feed ionizes by gaining electrons. The oxygen ions are transported through the membrane to an anode side in which the oxygen ions lose electrons and reconstitute into elemental oxygen.
The electrons, in case of an oxygen transport membrane formed by what is known as a mixed conducting material, are transported through the membrane from the anode side to the cathode side to ionize the oxygen. In other materials known as ionic conductors, the material is capable of conducting oxygen ions only and as such, electrodes and a separate electrical pathway are provided to conduct the electrons.
The separation of the oxygen from the oxygen containing feed can be driven by a partial pressure differential of oxygen between the cathode and anode sides of the oxygen transport membrane. Oxygen transport membranes can be used in reactors that are designed to separate oxygen. In such reactors, a pressurized process gas, for instance, air contacts the cathode side of the membrane to create the oxygen partial pressure differential between the cathode and anode side of the membrane. The partial pressure differential can also be created or enhanced with the use of a sweep gas, for instance, steam, that can be introduced to the anode side of the membrane to sweep away permeated oxygen and thereby lower the partial pressure differential on the anode side of the membrane.
In situations in which a sweep gas is used, the membrane can be a tubular membrane, closed at one end, with lance tubes projecting into the membrane. The oxygen containing feed gas, under pressure is introduced to the outer cathode side of the membrane and a sweep gas such as steam is introduced into the interior, anode side of the membrane to “sweep” away the permeated oxygen. Alternatively, the oxygen containing gas can be introduced into the inside of the membrane with or without the use of a lance tube and a reactant gas can be consumed on the anode side of the membrane to lower the oxygen partial pressure.
Oxygen transport membranes can be used to support reactions such as synthesis gas reactions. In such reactions, a fuel is reacted with the oxygen containing gas over a suitable catalyst provided at the anode side of the membrane to produce a hydrogen and carbon monoxide containing synthesis gas mixture. In such an application, the oxygen transport membrane is not only operated at elevated temperature, for instance near 1000° C. but also, separates a high pressure reacting gas, such as methane at 200 psig, from a lower pressure oxidant gas, such as air at 20 psig. These extreme operating conditions can result in premature failure of the oxygen transport membrane. Moreover, the failure itself can be catastrophic due to the brittle nature of the ceramic mixed conducting material, for instance, a perovskite or pseudo-perovskite.
In many reactor designs, the oxygen transport membranes are in the form of known closed end tubes. The tubes are attached to tubesheets such as illustrated in U.S. Pat. No. 5,820,655. Upon failure of the oxygen transport membrane, the higher pressure process gas will mix with the lower pressure process gas. In case of synthesis gas production, as described above, the processes gases can be a high pressure fuel stream, that upon failure of the oxygen transport membrane, can then mix with an oxygen containing feed with potentially catastrophic results. Therefore, upon failure of a membrane element, the reactor must be shut down and the failed oxygen transport membrane element must be replaced. This is a time consuming process that makes the use of such reactors unattractive.
As will be discussed, the present invention provides an isolation device for isolating a broken oxygen transport membrane element to avoid the need of immediate replacement.